Clutch assemblies have been used in automatic transmissions for vehicles for many years. As illustrated in FIG. 6, the typical clutch design 110 includes a first member 112 and a second member 114 rotatable relative to the first member. A clutch pack 116 including at least one first clutch disk 118 attached to the first member 112 and at least one second clutch disk 120 attached to the second member 114 is provided for selectively, frictionally engaging the first and second members 112, 114. In an automotive transmission, the first and second members 112, 114 can be any one of a rotating shaft, gears, and planetary gearing system components, or a fixed non-rotatable member, such as a housing. In a typical friction clutch assembly, an apply piston 122 is disposed in a fluid chamber 124 for selectively applying axial pressure on the clutch pack 116. A return spring mechanism 126 typically in the form of a spring pack, wave spring, or Bellville spring is used to apply a biasing force against the apply piston to bias the apply piston to a disengaged position. The input into the system is hydraulic pressure delivered to the piston chamber 124 and acting directly on the apply piston 122. The apply piston 122 translates toward the friction plates 118, 120 coming into contact with the plates and applying pressure thereto. The pressure applied to the friction plates 118, 120 increases and eventually causes the rotation of the component for which the system is design to engage.
The apply piston pressure must overcome the force of the return spring 126 in order to apply pressure to the clutch pack. The return spring's main function is to return the apply piston into the disengaged position from which it came after the apply pressure has dissipated in order to disengage the clutch.
The apply pressure applied to the clutch pack is dependent upon the fluid line pressure generated by the pump of a hydraulically controlled transmission and the surface area of the apply piston. Because of the space constraints on the design of a transmission, the size of the apply piston is greatly limited. Similarly, the pump of the hydraulically controlled transmission may also be limited due to size and weight requirements. Accordingly, it is desirable to provide a clutch design that increases the apply forces achievable with the clutch without greatly affecting the size of the system or requiring increased line pressure.